422 EEPOBT— 1882. 



prove stiffer than a suspension bridge ; but it is not so obvious tbat it 

 would also be cheaper. Careful comparative estimates have, however, 

 proved this to be so in the case of the Forth Bridge, and the reason is 

 not far to seek. In a long span bridge the weight of the structure itself 

 constitutes the chief portion of the load, whilst the pressure of the wind 

 is at least as important an element as the rolling load itself, to carry 

 which is the sole useful mission of the bridge. In a properly designed 

 continuous girder for a long span bridge the mass of metal will be 

 concentrated near the piers, where it will act with the smallest leverage 

 and produce the least bending moment. In an ordinary suspension bridge, 

 with stiffening girder vertically to provide for the rolling load, and 

 horizontally to meet wind stresses, the mass of metal will be somewhat 

 greater towards the centre of the bridge than at the piers, and conse- 

 quently for a given mass the moment will be much less in the continuous 

 girder than in the suspension bridge. Thus the Forth Bridge super- 

 structure weighs but 2 tons per foot run at the centre of the 1,700 feet 

 span, and 13-^ tons per foot run at the piers ; whilst in a suspension 

 bridge, as already stated, the weight of superstructure per lineal foot 

 would be somewhat greater at the centre than at the piers. This con- 

 sideration, coupled with the facts that suspension links are more costly 

 than girder work, that a suspension bridge requires a very costly anchor- 

 age, and that the contingencies and risks during erection in a stormy 

 estuary are very great, explains why, in such a case as the Forth Bridge, 

 well-designed continuous girders form a cheaper, as well as a far stiffer, 

 structure than a suspension bridge with stiffening girder. 



Continuous girdei's, as a rule, are made of uniform depth, and it 

 has been considered by some engineers a rather strained application of 

 the term to so describe the Forth Bridge girders. But clearly it is the 

 nature of the internal stresses, and not the external appearance of the 

 girder, which should decide the question ^ and from, this point of view 

 the proposed Forth Bridge is in the strictest sense of the words a con- 

 tinuous girder bridge. By all authorities a beam is considered to be 

 continuous if it is either rigidly or partially fixed as well as supported at 

 each end in such a manner that a pair of equal and opposite couples act 

 on the vertical planes at its points of support. In the case of the Forth 

 Bridge, such continuity is attained by connecting together the ends of 

 the two 1,700 feet spans at Inchgarvie, and by projecting the other ends 

 a distance of 675 feet beyond the main piers, and weighting them to the 

 required extent. The moment of the couples and the position of the 

 points of contrary flexure in a continuous girder may be regulated at will, 

 either by putting an initial stress on the girder or by severing either the 

 top or bottom member at the desired point. In the case under con- 

 sideration, the latter method has been adopted, and the question of the 

 most advantageous position for the points of contrary flexure was a sub- 

 ject of elaborate investigation, as it was known to have a vital influence 

 on the economy of the design. Having reference to all the conditions of 

 the problem, it proved to be most advantageous to fix the points at a 

 distance of 675 feet from the piers, so that in effect the 1,700 feet girder 

 may be considered as made up of two cantilevers each 675 feet in length, 

 and a central girder 850 feet in span. 



Similarly, on investigation, the most generally advantageous depth 

 proved to be about 60 feet at the centre, and 350 feet at the piers ; and, 

 this being settled, the next thing to be determined was the most advan- 



